The present invention relates to the effective capture and removal of particulate material from dust laden air streams that must exit to atmosphere from many different process systems. Process systems include such varied applications as power plants, process kilns, cement plants, grain processing plants, foundries, steel mills, hot mix asphalt plants and many other industrial processes.
It is generally known and accepted throughout these industries that fabric filter collection systems (“bag houses”) provide the most economical and efficient “dry method” of removing small particulate material from process air streams. EPA air emissions laws governing the amount of particulate emissions allowed to exit into the atmosphere from any given process are now generally standardized throughout the United States and Canada. Additionally, these laws have remained reasonably stable for the past several years. The current accepted emissions level for particulate material permitted to atmosphere is “0.04 grains per dry standard cubic foot of exhaust air”. This law is commonly referenced by EPA agencies as 0.04 gdscf. In order to achieve an atmospheric loading of 0.04 gdscf or less, it is usually necessary to utilize fabric filters and achieve a cleaning efficiency of 0.99895% or greater.
Bag houses are manufactured in many sizes and configurations. Bag houses are typically available in “portable mode” or “stationary mode”. A portable bag house is limited in size by the availability of permits and the ability to transport the bag house on a highway. In contrast, stationary units are offered in almost any size with sections assembled on site to accommodate a specific process.
A standard bag house consists of a vessel, generally a rectangular container, having a dirty air inlet on one end leading into a lower dirty air section with a clean air outlet on the other end leading from an upper clean air section. Incoming dirty air must pass thorough the bag cloth in order to reach the clean air outlet.
A typical bag house contains a multitude of woven fabric bags that can be of varying types and sizes (typically 6″ diameter×12′0″ long). The filter bags are generally hung or mounted on a bag tube sheet by a snap band collar at the open end of the bag which fits tightly into holes in the tube sheet at the top of the bag section, thus providing air tight seals. Each fabric membrane bag is fitted over a separate wire cage or extruded cage. The cages provide support for the bag filter that allows the bag filter to remain open and to assume a specific shape during cleaning operations.
As the dirty air passes through the filter bags, particulate material from the dirty air stream is left on the outside of the cloth surface. As the dust accumulates and a dust cake forms on the outside of the bag, the dust cake becomes the actual filtration membrane. As the dust cake thickness increases, the airflow resistance through the filter media also increases. When a certain level of pressure drop (delta P resistance) is reached, the dust cake must be reduced in order to restore adequate airflow to the system.
There are a number of accepted cleaning methods used to “drop” the dust cake. These cleaning methods include compressed air pulse, vibration, shaking, reverse air blowing, and atmospheric venting. The most commonly used of these cleaning methods is the compressed air pulse method. In this method, a momentary burst of high-pressure compressed air is forced down into the throat of each bag, sending a shock wave down the length of the bag that dislodges the dust cake. The dust that is removed from the outer bag surface drops by gravity to a hopper collection area in the bottom of the bag house. The accumulated dust is then typically removed by a screw auger system.
Despite the widespread use of traditional bag houses, the science of particulate collection still suffers from shortcomings. During the operation of a typical bag house, cleaned air is inadvertently recycled through the system. Generally, air from the filter bags nearest the dirty air inlet flows in the correct direction. However, when the clean air from the bags closest to the dirty air inlet passes over the filter bags near the clean air outlet of the system, some of the clean air is drawn back down into the filter bags and passes through the filter bags in the wrong direction. This creates a re-circulation problem that reduces the efficiency of the bag house.
It has been determined that current methods of bag house design, as well as current methods of bag cleaning, are grossly inefficient. Additionally, in some iristances the inefficiencies in bag house design and bag cleaning can lead to negative air effects that actually contribute to air emissions. Research by experts in the areas of computational fluid dynamics (CFD), as well as fluid dynamics, heat transfer, thermodynamics, and aerodynamics has confirmed these inefficiencies.
Computational fluid dynamics (CFD) is concerned with obtaining numerical solutions to fluid flow problems by using computers. Equations governing the fluid flow problem include conservation of mass, Navier-Stokes (conservation of momentum), and the energy equation. These equations form a system of coupled non-linear partial differential equations (PDEs). Because of the non-linear terms in these PDEs, analytic methods can yield very few solutions. In general, closed form analytic solutions are possible only if these PDEs can be made linear, either because non-linear terms naturally drop out (e.g., fully developed flows in ducts and flows that are inviscid and irrotational everywhere) or because nonlinear terms are small compared to other terms so that they can be neglected (e.g., creeping flows, small amplitude sloshing of liquid, etc.). If the non-linearities in the governing PDEs cannot be neglected, which is the situation for most engineering flows, and then numerical methods are needed to obtain solutions. With CFD the differential equations governing the fluid flow are replaced with a set of algebraic equations. This process is called discretization. Once the equations are discretized they can be solved with a digital computer to get an approximate solution.
The discretized equations are solved on a computational grid. The equations are parallelized using a Message Passing Interface (MPI) technique that allows the computational grid to be divided up onto different computers. The computers work together to solve the equations simultaneously, greatly decreasing the time required to achieve a solution. This technique was used to model the fluid flow inside of bag houses. The computer modeling was conducted over a period of 45 days using 64 computers. The computational grid consisted of 2 million grid points that corresponds to 12 million equations. Several overall designs and different types of inlet and outlet locations and orientations were tested.
Research discovered that an internal re-circulation phenomenon was present in all tested, existing bag house designs. The re-circulation phenomenon is related to the flow patterns in the bag house. The modeling showed that the direction of flow through the bags depend on their location in the bag house. For bags that are far from the clean air plenum outlet the air flows in an upward direction, the correct direction for filtering. However, in bags that are near the clean air plenum outlet the air moves in a reversed direction that prevents the bag from filtering. This also indicates that the air involved in the re-circulation phenomenon becomes re-contaminated and must again be cleaned in order to exit the unit to atmosphere.
The discovery of the re-circulation phenomenon tells us that current bag houses must be grossly oversized (4.0 to 1.0 air to cloth ratio) to compensate for the number of bags that are engaged in negative re-circulation. These bags do not contribute to the filtration process and can therefore only reduce bag house overall capacity while reverse flowing rather than online cleaning. There is also considerable evidence that the re-circulation phenomenon allows very fine dust that continuously migrates through micro pores of the filter cake to accumulate inside the bags during re-circulation. Then, this fine dust is lifted and emitted in periodic bursts to the atmosphere when disturbed by the shock pulse jet air cleaning cycle. These periodic bursts of fine dust during bag house stack tests can result in failure to pass the particulate loading and opacity air tests. This problem is relatively common with pulse jet type cleaning systems. The reverse flow of re-circulating cleaned air back into the interior of the bags is actually caused by a combination of the imbalance of vacuum exerted on the exit air plenum (vacuum influence by exhaust fan) and the internal bag section vortex vacuum influence generated inside of the dirty air entry plenum. The flow field in the bag house is complex and highly three-dimensional.
In the hopper region below the bags high air velocity can create strong vortices that influence the flow pattern through the bags. These vortices can create low-pressure regions below the bags that overcome the vacuum influence of the clean air plenum. This in turn can cause reverse flow through the bags. It is also reasonable to assume that the reverse flow phenomenon moves around the bag house and will change from bag to bag as the resistance changes due to dust cake loading and cleaning. The re-circulation phenomenon is further influenced by the relative position and configuration of inlet and outlet plenums and how they affect internal air flows, as well as the location, size and orientation of inlet blast plates.
Needs exist for bag houses that eliminate the re-circulation problem and allow for maximum air cleansing potential and effective removal of dust cakes.